Mayfly Species Stenonema mexicanum (Cream Cahills)

A small species of Stenonema pulchellum group, allied to S. integrum (now a synonym of Stenonema mexicanum); dark streaks in spiracular areas; hind wing not dark-bordered; tails dark-ringed.

Head pale whitish; a small black dot at inner lower corner of eye, opposite antenna; narrow reddish line below antenna; antennae wholly pale. A V-shaped reddish mark on vertex between lateral ocelli; on each side of head near posterior margin, a large dusky triangular spot. Thorax creamy white; an oblique purplish red streak laterally on pronotum, its posterior end continued laterally as a reddish line parallel to hind margin; a black line in postero-lateral angle. Oblique purplish black line on pronotal pleura extending to coxa of fore leg. Anterior and posterior to middle and hind legs, and extending up between them, are blackish pencilings on pleura. Oblique purplish marks laterally on metanotum. Legs pale yellowish white, femora yellowish; fore femur with tinges of rose; all femora with median and apical purplish rose bands. Apex of fore tibia faintly dusky; claws pale brownish yellow; at base of each tibia, a purplish rose streak. Wings hyaline. Costa, subcosta and radius yellow, other longitudinal veins pale. Cross veins of fore wing dark brown, paler in anal region. No reddish stigmatic stain. A slight crowding of cross veins at bulla, 2 in each of first three spaces. Little tendency for serial arrangement of cross veins, as in S. integrum (now a synonym of Stenonema mexicanum). Hind wing wholly pale, veins colorless, outer margin not darkened.

Abdominal segments 1-7 hyaline white. Posterior margins of all tergites narrowly purplish black, not restricted to dorsal area. Extending forward from this dark margin on each side is a purplish black oblique streak, in place of the more usual spiracular dot. A bifurcate purplish streak on posterior portion of mid-dorsal line on tergites 3 and 6, and traces of this dark shading on 2 and 7; a black median dot at posterior margin of 4. Segments 8-10 opaque, yellowish. Genitalia much as in S. integrum (now a synonym of Stenonema mexicanum); forceps relatively longer. Tails white; alternate joinings narrowly purplish black.

Described as S. integrum subspecies integrum

Body length 5 mm, wing length 6 mm

A small species of Stenonema pulchellum group; spiracular dots present; tails dark-ringed; outer margin of hind wing dark.

Head and thorax yellowish or whitish clay-colored; antennae pale whitish; ocelli black-ringed at base. Fore leg yellowish, femur with faint reddish tinge; tibia black at tip; tarsal joinings narrowly blackish. Basal tarsal joint fully one-half, often slightly more than one-half of second joint. All femora with median and apical reddish bands, sometimes indistinct on hind leg. All claws dark purplish. Wings hyaline; longitudinal veins fine, pale yellowish. Cross veins “thicker and distinctly dark, comparatively few in number, especially in the basal area, only two or three on costa before the bulla, where there is a group of four or five; below this the cross veins of the radial sector and median areas form an irregular line across the wing, preceded by long elongate cells” (McD.). A distinct reddish stain in stigmatic area, in subcostal space. Veins of hind wing hyaline; outer margin narrowly but distinctly brownish.

Abdominal segments 1-7 hyaline white; posterior margins of tergites very narrowly blackish dorsally; traces of a purplish black streak on posterior half of mid-dorsal line, on tergites 3 and 6. A short black mark (rather a streak than a dot) on each tergite laterally, at end of black border on posterior margin. Segments 8-10 opaque, yellowish white. Genitalia rather similar to pulchellum type, but lateral portions of penes shorter and rounder, the whole being less L-shaped in appearance. Tails whitish, joinings narrowly dark brown. Distinguished from pulchellum and its immediate allies by the dark-margined hind wings and the fewer cross veins, which tend to be crowded at bulla and arranged in series beyond and below this point. Distinguished from its near ally, Stenonema exiguum, by the presence of dark spiracular marks and dark tail joinings; also from bellum (now a synonym of S. mexicanum) by the dark margin of the hind wing and the reddish stigmatic stain.